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Abstract:

A sensing apparatus comprises an excitation source configured to induce
waves in a target, and a fiber optic interferometer configured to sense
the induced waves in the target. The fiber optic interferometer comprises
a probe segment having a probe segment end, and an adjustable coupler
configured to permit setting a gap between the probe segment end and the
target. A controller is coupled to the adjustable coupler and configured
to set the gap between the probe segment end and the target.

Claims:

1. A sensing apparatus comprising: an excitation source configured to
induce waves in a target; and a fiber optic interferometer configured to
sense the induced waves in the target and comprising a probe segment
having a probe segment end, an adjustable coupler configured to permit
setting a gap between the probe segment end and the target, and a
controller coupled to said adjustable coupler and configured to set the
gap between the probe segment end and the target.

2. The sensing apparatus of claim 1, wherein said fiber optic
interferometer further comprises: a laser source; a photodetector coupled
to said controller; and an optical coupler operatively connecting said
laser source, said photodetector, and said probe segment.

3. The sensing apparatus of claim 1, wherein said controller is further
configured to generate target data based upon the sensed induced waves.

5. The sensing apparatus of claim 1, wherein said adjustable coupler
comprises a sleeve surrounding said probe segment end, and at least one
of a heating source and a codling source associated therewith.

7. The sensing apparatus of claim 1, wherein said adjustable coupler
comprises a sleeve surrounding said probe segment end and a biasing
member configured to urge said sleeve in contact with the target.

8. The sensing apparatus of claim 1, wherein said probe end comprises an
optical fiber with an angled endface.

10. A sensing apparatus comprising: an excitation source configured to
induce waves in a target; a fiber optic interferometer configured to
sense the induced waves in the target and comprising a laser source, a
probe segment having a probe segment end, an adjustable coupler
configured to permit setting a gap between the probe segment end and the
target, a controller coupled to said adjustable coupler and configured to
set the gap between the probe segment end and the target, a photodetector
coupled to said controller, and an optical coupler operatively connecting
said laser source, said photodetector, and said probe segment; said
controller also configured to generate target data based upon the sensed
induced waves.

12. The sensing apparatus of claim 10, wherein said adjustable coupler
comprises a sleeve surrounding said probe segment end and at least one of
a heating source and a cooling source associated therewith.

14. A method of sensing a target comprising: inducing waves in the target
using an excitation source; and sensing the induced waves in the target
using a fiber optic interferometer comprising a probe segment having a
probe segment end by at.least setting a gap between the probe segment end
and the target using an adjustable coupler.

15. The method of claim 14, wherein setting the gap comprises using a
controller coupled to the adjustable coupler to set the gap between the
probe segment and the target.

16. The method of claim 15, further comprising generating target data
based upon the sensed induced waves, using the controller.

17. The method of claim 14, wherein the fiber optic interferometer
further comprises: a laser source; a photodetector; and an optical
coupler operatively connecting the laser source, the photodetector, and
the probe segment.

19. The method of claim 14, wherein the adjustable coupler comprises a
sleeve surrounding the probe segment end and at least one of a heating
source and a cooling source associated therewith.

20. The method of claim 19, wherein the heating source comprises a laser.

21. The method of claim 14, wherein the adjustable coupler comprises a
sleeve surrounding the probe segment end and a biasing member configured
to urge the sleeve in contact with the target.

22. A sensing apparatus comprising: an excitation source configured to
induce waves in a target; a fiber optic interferometer configured to
sense the induced waves in the target and comprising a laser source, a
probe segment having a probe segment end to be positioned adjacent the
target and defining a gap therebetween, a optical coupler operatively
connecting said laser (source and said probe segment, and a controller
coupled to said laser source and configured to adjust a wavelength of
said laser source so that a desired multiple of the wavelength equals the
gap between the probe segment end and the target.

23. The sensing apparatus of claim 22, wherein said fiber optic
interferometer further comprises a photodetector coupled to said
controller; and wherein said optical coupler also operatively connects
said photodetector to said probe segment and said controller.

24. The sensing apparatus of claim 22, wherein said controller is further
configured to generate target data based upon the sensed induced waves.

25. The sensing apparatus of claim 22, wherein said probe end comprises
an optical fiber with an angled endface.

27. A sensing apparatus comprising: an excitation source configured to
induce waves in a target; a fiber optic interferometer configured to
sense the induced waves in the target and comprising a laser source, a
probe segment having a probe segment end comprising an optical fiber with
an angled endface to be positioned adjacent the target and defining a gap
therebetween, a optical coupler operatively connecting said laser source
and said probe segment, and a controller coupled to said laser source and
configured to adjust a wavelength of said laser source so that a desired
multiple of the wavelength equals the gap between the probe segment end
and the target; said controller also configured to generate target data
based upon the sensed induced waves.

28. The sensing apparatus of claim 27, wherein said fiber optic
interferometer further comprises a photodetector coupled to said
controller; and wherein said optical coupler also operatively connects
said photodetector to said probe segment and said controller.

30. A method of sensing a target using a fiber optic interferometer
comprising a laser source, a probe segment having a probe segment end to
be positioned adjacent the target and defining a gap therebetween, and a
optical coupler operatively connecting said laser source and said probe
segment, the method comprising: inducing waves in the target using an
excitation source; sensing the induced waves in the target using the
fiber optic interferometer; adjusting a wavelength of said laser source
so that a desired multiple of the wavelength equals the gap between the
probe segment end and the target using a controller.

31. The method of claim 30, wherein the fiber optic interferometer
further comprises a photodetector coupled to the controller; and wherein
the optical coupler also operatively connects the photodetector to the
probe segment and the controller.

32. The method of claim 30, further comprising generating target data
based upon the sensed induced waves using the controller

33. The method of claim 30, wherein the probe end comprises ar optical
fiber with an angled endface.

34. The method of claim 30, wherein the excitation source comprises at
least one pulsed laser.

Description:

FIELD OF THE INVENTION

[0002] The present invention relates to the field of interferometers and,
more particularly, to fiber optic interferometers and related methods.

BACKGROUND OF THE INVENTION

[0003] Ultrasonic waves may be used to probe a variety of materials,
particularly for thickness gauging and flaw detection. The ultrasonic
waves are typically generated with a piezoelectric transducer. The
ultrasonic waves propagate through the material, reflecting from
interfaces (in thickness gauging applications) or internal features (in
flaw detection applications). The scattered ultrasonic waves propagate
back to the surface of the material, causing the surface to vibrate at
the ultrasound frequency. This vibration may be detected with a
piezoelectric transducer similar to the one used to generate the
ultrasonic waves, and then analyzed to generate data about the material.

[0004] Optical detection techniques can be used in place of the
piezoelectric transducers to remotely detect the ultrasonic waves.
Generally, a laser probe beam is directed onto the material. When the
surface vibrates it imparts a phase shift onto the reflected beam. This
phase shift is detected with a photodetector after mixing the reflected
probe beam with a stable reference beam and measuring the amplitude and
frequency or phase of the photodetector output intensity fluctuations.
The reference beam originates from the same laser source as the reflected
probe beam, and the output signal from the photodetector corresponds to
the surface motion.

[0005] One problem with laser detection systems is low sensitivity.
Typically, the material surface that is being probed has a diffusely
reflecting or scattering quality. Consequently, the reflected beam is
highly aberrated and its wavefront is mismatched with respect to the
reference beam. The resulting signal produced by the photodetector is
therefore weak and lacks precision.

[0006] In U.S. Pat. No. 6,075,603 to O'Meara, a contactless system for
imaging an acoustic source within a workpiece is disclosed. In this
system, an array of discrete optical detectors are arranged in a pattern.
A probe beam is directed onto a vibrating surface in a pattern that
corresponds to the detector array. The probe beam is reflected onto the
detector array and a reference beam is also directed onto the detector
array at an angle to the probe beam to produce fringe patterns on the
detectors that correspond to the surface vibration pattern. A readout
system utilizes the discrete detector outputs to produce an array output
signal indicative of at least a size and two dimensional location for the
acoustic source relative to the vibrating surface. This system, however,
may not provide the desired accuracy, and may be sensitive to
fluctuations in the length of the paths between the probe beam and the
surface, and the reference beam and the surface.

[0007] U.S. Pat. No. 7,262,861 to Pepper discloses a laser ultrasonic
inspection apparatus which enables remote sensing of thickness, hardness,
temperature and/or internal defect detection. A laser generator impinges
a workpiece with light for generating a thermo-elastic acoustic reaction
in a workpiece. A probe laser impinges the workpiece with an
annularly-shaped probe light for interaction with the acoustic signal in
the workpiece resulting in a modulated return beam. A photodetector
having a sensitive region is for detecting an annularly-shaped fringe
pattern generated by an interaction of a reference signal with the
modulated return beam at the sensitive region.

[0008] This system, however, may not provide the desired accuracy, and may
be sensitive to fluctuations in the length of the path between the probe
beam and the surface, or fluctuations in the path lengths of the
reference and measurement arms of the interferometer.

SUMMARY OF THE INVENTION

[0009] In view of the foregoing background, it is therefore an object of
the present invention to provide a sensing apparatus. This and other
objects, features, and advantages in accordance with the present
invention are provided by a sensing apparatus permitting adjustment of a
gap between an interferometer probe and a target that includes an
excitation source configured to induce waves in a target, and a fiber
optic interferometer configured to sense the induced waves in the target.
The fiber optic interferometer comprises a probe seyment having a probe
segment end, and an adjustable coupler configured to permit setting a gap
between the probe segment end and the target. A controller is coupled to
the adjustable coupler and configured to set the gap between the probe
segment and the target.

[0010] The fiber optic interferometer may also include a laser source, and
a photodetector coupled to the controller. A optical coupler operatively
connects the laser source, the photodetector, and the probe segment. The
controller is further configured to generate target data based upon the
sensed induced waves.

[0011] Setting the gap between the probe segment and the target
advantageously allows the fiber optic interferometer to be tuned such
that the distance between the probe segment end and the target is a
desired fraction or multiple of the wavelength of light emitted from the
probe segment end. This helps to minimize distortions in the generated
target data.

[0012] In some applications, the adjustable coupler may comprise a
piezoelectric body. Alternatively, the adjustable coupler may comprise a
sleeve surrounding the probe segment end and at least one of a heating
source and a cooling source associated therewith. In some applications,
the heating source may be a laser.

[0013] The adjustable coupler comprises a sleeve surrounding the probe
segment end. In addition, the adjustable coupler may further comprise a
biasing member configured to urge the sleeve in contact with the target.
The probe end comprises an optical fiber with an angled endface, and the
excitation source may comprise at least one pulsed laser.

[0014] A method aspect is directed to a method of sensing a target
comprising inducing waves in the target using an excitation source, and
sensing the induced waves in the target using a fiber optic
interferometer comprising a probe segment end by at least setting a gap
between the probe segment end and the target.

[0015] According to another aspect, a sensing apparatus comprises an
excitation source configured to induce waves in a target, and a fiber
optic interferometer configured to sense the induced waves in the target.
The fiber optic interferometer comprises a laser source, and a probe
segment having a probe segment end to be positioned adjacent the target
and defining a gap therebetween. A optical coupler is operatively
connecting the laser source and the probe segment. In addition, a
controller coupled is to the laser source and configured to adjust a
wavelength of the laser source so that a desired multiple of the
wavelength equals the gap between the probe segment end and the target.

[0016] According to a further aspect, a sensing apparatus includes an
excitation source configured to induce waves in a target, and a fiber
optic interferometer configured to sense the induced waves in the target.
The fiber optic interferometer comprises a laser source, and a probe
segment having a probe segment end to be positioned adjacent the target
and defining a gap therebetween. An optical coupler operatively connects
the laser source and the probe segment, and a controller is coupled to
the laser source and configured to adjust a wavelength of the laser
source so that a desired multiple of the wavelength equals the gap
between the probe segment end and the target.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] FIG. 1 is a schematic block diagram of a sensing apparatus,
according to the present invention.

[0018]FIG. 2 is a schematic sectional view of an adjustable coupler, as
used with the sensing apparatus of FIG. 1.

[0019]FIG. 3 is a schematic sectional view of another adjustable coupler,
such as may be used with the sensing apparatus of FIG. 1.

[0020] FIG. 4 is a schematic sectional view of yet another adjustable
coupler, such as may be used with the sensing apparatus of FIG. 1.

[0021] FIG. 5 is a schematic cross sectional view of an additional
adjustable coupler, such as may be used with the sensing apparatus of
FIG. 1.

[0022] FIG. 6 is a schematic block diagram of another embodiment of a
sensing apparatus, according to the present invention.

[0023]FIG. 7 is a flowchart of a method of sensing a target in accordance
with the present invention.

[0024] FIG. 8 is a partial schematic sectional view of an adjustable
coupler of a biological sensing apparatus sensing an arterial wall in
accordance with the present invention.

[0025] FIG. 9 is a partial schematic sectional view of an adjustable
coupler of a material inspection apparatus sensing a weld in accordance
with the present invention.

[0026] FIG. 10 is a schematic block diagram of another sensing apparatus
in accordance with the present invention.

[0027] FIG. 11 is a schematic block diagram of yet another sensing
apparatus in accordance with the present invention.

[0028] FIG. 12 is a flowchart of another method of sensing a target in
accordance with the present invention.

[0029] FIG. 13 is a partial schematic sectional view of a biological
sensing apparatus sensing an arterial wall in accordance with the present
invention.

[0030] FIG. 14 is a partial schematic sectional view of a material
inspection apparatus sensing a weld in accordance with the present
invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] The present invention will now be described more fully hereinafter
with reference to the accompanying drawings, in which preferred
embodiments of the invention are shown. This invention may, however, be
embodied in many different forms and should not be construed as limited
to the embodiments set forth herein. Rather, these embodiments are
provided so that this disclosure will be thorough and complete, and will
fully convey the scope of the invention to those skilled in the art. Like
numbers refer to like elements throughout, and prime notation and
multiple prime are used to indicate similar elements in alternative
embodiments.

[0032] Referring initially to FIG. 1, a sensing apparatus 10 in accordance
with the present invention is now described. The sensing apparatus 10 is
used to sense, or determine, a variety of properties of a target 12,
including, for example, the dimensions of the target, the material
composition of the target, and the thickness of the target.

[0033] The sensing apparatus 10 includes an excitation source 14,
illustratively a pulsed laser, configured to induce ultrasonic waves in
the target 12. The excitation source 14 is an optical source and is
illustratively coupled to the target 12 via an optical fiber 15 having an
end portion 16 in physical contact with the target, although it should be
appreciated that in some embodiments the excitation source is not coupled
to the target via an optical fiber but rather radiates the target via
free space. The excitation source 14 induces the ultrasonic waves in the
target 12 by rapidly heating it. It should be appreciated that in some
applications, the excitation source 14 may be a broadband optical source,
or a doped fiber amplifier.

[0034] A fiber optic interferometer 17 senses the induced waves and
generates target data based thereupon. In particular, the fiber optic
interferometer 17 comprises a probe segment 30 having a probe segment end
31 coupled to the target 12. The interferometer laser source 18 is
connected to an adjustable coupler 14 via optical fibers 21 and through
an optical isolator 19 and an optical coupler 22. Also coupled to the
optical coupler 22 is a photo-detector 20.

[0035] The adjustable coupler 24 is in physical contact with the target
12, and permits setting a gap between the probe segment end 31 and the
target. A controller 26 is coupled to the adjustable coupler 24 and is
configured to control the adjustable coupler to thereby set the gap
between the probe segment end 31 and the target 12.

[0036] Operation of the fiber optic interferometer 17 is now described.
The interferometer laser source 18 radiates the target 12 via the probe
segment end 31. A portion of the light radiating within the probe segment
30 is reflected back as it hits the probe segment end 31, through the
optical coupler 22, and into the photodetector 20. Similarly, a portion
of the light radiating within the probe segment 30 is radiated from the
probe segment end 31 onto the target 12. This light is then reflected
from the target 12 back into the probe segment 30 via the probe segment
end 31, through the optical coupler 22, and into the photodetector 20.
Consequently, the light reflected from the probe segment end 31 and the
light reflected from the target 12 will combine, and the superposition
thereof is detected by the photodetector 20. The light reflected by the
target will typically undergo a phase change due to the ultrasonic waves
and resulting vibrations in the target 12, and therefore will have a
different phase than the light reflected by the probe segment end 31,
causing constructive and destructive interference to occur therebetween.
This interference therefore reflects a detection of the sensed induced
waves and can be analyzed in order to determine various properties of the
target, as will be appreciated by those skilled in the art.

[0037] The controller 26 generates target data based upon the sensed
induced waves. To do so, a laser pulse from the pulsed laser 14 triggers
the start of a measurement cycle, performed by the controller 26, in the
time domain. Signal peaks observed by the controller 26 correspond to the
transmit time of surface waves from the point of excitation (that is, the
point of the target 12 on which the pulse from the pulsed laser 14
radiates) to the probe segment end 31. Since the distance between the
excitation point and the probe segment end 31 is known, the acoustic
velocity of the ultrasonic waves in the target 12 can be calculated. By
comparing this acoustic velocity to a table of acoustic velocity for
different materials, the material composition of the target can be
determined. It should be understood that the adjustable coupler 24 and
excitation source probe 16 can be scanned to different locations on the
target 12, so as to gather information about many points of the target.

[0038] The controller 26 may include a processor and a memory cooperating
therewith. The memory may be volatile or non-volatile, and the processor
may be an integrated circuit, in some applications.

[0039] In addition, the controller 26 performs typical interferometric
calculations as known to those of skill in the art on the superposition
of the light reflected by the probe segment end 31 and the light
reflected by the target 12 to potentially determine the dimensions and/or
the thickness of the target. Since a difference in the length of the path
traveled by the light reflected by the probe segment end 31' and the
light reflected by the target 12 will result in an additional phase
difference therebetween, it is desirable for the difference in the length
of that path to remain the same. That is, it desirable for the gap
between the probe segment end 31 and the target 12 to remain constant,
such that the gap is a desired multiple of the wavelength of the light
radiated by, and reflected into, the probe segment end 31'. The multiple
used need not be an integer in some embodiments, and need not be greater
than one in some embodiments.

[0040] As stated above, the controller 26 controls the adjustable coupler
24 to adjust the gap. As shown in FIG. 2, the adjustable coupler 24, in
some embodiments, may comprise a sleeve 29 surrounding probe segment end
31, and a biasing member 28 to urge the sleeve 29 in physical contact
with the target 12. The biasing member 28 comprises a cylinder configured
to receive the sleeve 29, and a spring arranged so as to urge the sleeve
in contact with the target 12. A ferrule 35 slidably holds the probe
segment end 31 inside the sleeve 29. The purpose of the biasing member 28
urging the sleeve 19 in contact with the target 12 is to help coarsely
adjust the gap between the probe segment end 31 and the target 12 even
though the target may be vibrating.

[0041] Thermal drifting, however, may cause the sleeve 19, the probe 30,
and the probe segment end 31 to expand and contract at different rates,
which leads to the gap changing. Since this is not desirable, the
adjustable coupler 24 may include additional components to fine tune the
gap.

[0042] For example, as shown in FIG. 3, the adjustable coupler 24' may
include a piezoelectric sleeve 32' surrounding the probe segment end 32',
which is in turn surrounded by the sleeve 29'. The controller 26' applies
a voltage to the piezoelectric sleeve 32', causing the piezoelectric
sleeve to expand or contract, thereby altering the length of the probe
segment end 31'. This therefore allows fine tuning of the gap between the
probe segment end 31' and the target 12'. The controller 26' may be
coupled to the piezoelectric sleeve 32' via any suitable method, such as
suitable electrical contacts between the sleeve 19' and the piezoelectric
sleeve 32'.

[0043] Another embodiment of the adjustable coupler 24 is shown in FIG. 4,
and includes a temperature control unit 33'' surrounding the sleeve 19''.
The temperature control unit 33'' is illustratively a Peltier effect
unit, and is controlled by the controller 26''. The controller 26'' uses
the Peltier effect unit 24'' to heat or cool the sleeve 19'' and probe
segment end 31'' to thereby cause the sleeve 19'' and probe segment end
31'' to expand or contract, which in turn allows fine tuning of the gap
between the probe segment end and the target 12''.

[0044] A further embodiment of the adjustable coupler 24 is shown in FIG.
5, and includes a laser heating source 34''' configured to radiate the
sleeve 19''', and thereby heat the sleeve 19''' and probe segment end
31''' to cause the sleeve and probe segment end to expand or contract,
which in turn allows fine tuning of the gap between the probe segment end
and the target 12'''.

[0045] Referring once again to FIG. 1, in the above examples, it should be
understood the controller 26 controls the adjustable coupler 24 based
upon an error signal. This error signal may be the DC component of the
light detected by the photodetector 20, for example.

[0046] A further embodiment of the sensing apparatus 110 is shown in FIG.
6. Here, there is no mechanically adjustable coupler, although the
excitation source 114, optical isolator 119, optical coupler 122,
photodetector 120, probe segment 130, probe segment end 131, and optical
fibers 115, 121 are similar to those described above with reference to
FIG. 1. Rather than adjusting the gap between the probe segment end 131
and the target 112 such that the gap is a desired multiple of the
wavelength of the light radiated by, and reflected into, the probe
segment end, the wavelength of the interferometer laser source 118 is
adjusted by the controller 126 such that a desired multiple of the
wavelength equals the gap.

[0047] It should be understood that the sensing apparatuses 10, 10', 10'',
10''', 100 disclosed above may include an array of excitation sources 14,
14', 14'', 14''', 114, and an array of fiber optic interferometers 18,
18', 18'', 18''', 118.

[0048] With additional reference to the flowchart 40 of FIG. 7, a method
of sensing a target is now described. After the start (Block 41), waves
are induced in a target by an excitation source (Block 42). Next, the
induced waves are sensed by a fiber optic interferometer (Block 43). The
fiber optic interferometer comprises a probe segment having a probe
segment end, an adjustable coupler configured to permit setting a gap
between the probe segment end and the target.

[0049] Next, the method includes setting the gap between the probe segment
end and the target using a controller coupled to the adjustable coupler
(Block 44). Then, target data is generated based upon the sensed induces
waves, using the controller (Block 45). Block 46 indicates the end of the
method.

[0050] It should be understood that the sensing apparatuses 10, 10', 100
disclosed above offer numerous advantages. For example, the use of a
pulsed laser 14, 14', 114 as an excitation source allows a wide bandwidth
of ultrasonic waves to be induced in the target 12, 12', 112 as opposed
to conventional piezoelectric excitation sources which are only capable
of producing more narrow bandwidths. For, pxample, the pulsed laser 14,
14', 114 can produce ultrasonic waves with a bandwidth above 1 Mhz, which
is not possible with conventional piezoelectric excitation sources.

[0051] In addition, the ability of the sensing apparatuses 10, 10', 10'',
10''' to either adjust the gap between the probe segment end 31, 31',
31'', 31''' and the target 12, 12', 12'', 12''' or the wavelength of the
interferometer laser source 118, on the fly and based upon a feedback
error signal provides for precise results, as effects that negatively
impact the results can be adjusted for and mitigated. Furthermore, the
use of a pulsed laser 14, 14', 14'', 14''', 114 as the excitation source,
coupled with the use of the fiber optic interferoMeter 17, 17', 17'',
17''', 117 allows the sensing apparatus 10, 10', 10'', 10''', 110 to be
compact and portable. Moreover, the use of optical fibers to couple the
pulsed laser 14, 14', 14'', 14''', 114 and interferometer laser source
18, 18', 18'', 18''', 118 to the target 12, 12', 12'', 12''', 112 allows
the sensing of hard to reach targets, since the optical fibers may be
inserted into small spaces.

[0052] The sensing apparatuses 10, 10', 10'', 10''', 110 disclosed herein
are useful in a wide variety of applications. For example, they may be
useful in medical imaging systems, for sensing and imaging body parts.
For example, the optical fibers of the pulsed laser 14, 14', 14'', 14''',
114 and interferometer laser source 18, 18', 18'', 18''', 118 may be
inserted into arteries, in order to image those arteries or measure the
thickness thereof, or may be inserted into a trachea in order to image
various components of the digestive system of a patient. Shown in FIG. 8
is an embodiment where the sensing apparatus 200 (similar to the sensing
apparatuses disclosed above) is a biological sensing device, and the
target 212 is an artery having an arterial wall 250. Here, the controller
will generate anatomical data about the arterial wall 250, such as a
thickness or density of the arterial wall. Those skilled in the art will
appreciate that any biological sample or body part may be sensed using
this sensing apparatus 200.

[0053] In addition, the sensing apparatuses 10, 10', 10'', 10''', 110 may
be used for materials inspection. For example, small welds, or welds in
inaccessible places, may be inspected using the sensing apparatuses 10,
10', 10'', 10''', 110. Wire bonds in electronic devices may be inspected
using the sensing apparatuses 10, 10', 10'', 10''', 110. Hydraulic lines,
such as those used in avionics systems of aircraft, or brake lines of a
motor vehicle, may be inspected using the sensing apparatuses 10, 10',
10'', 10''', 110. Shown in FIG. 9 is an embodiment where the sensing
apparatus 300 (similar to the sensing apparatuses disclosed above) is a
material inspection device, and the target 312 is a workpiece having a
weld 350 to be inspected. Here, the controller will generate material
data about the material weld 350, such as a thickness, density, or
composition of the weld 350. Of course, this material inspection device
300 need not be limited to weld inspection and may be used to sense or
inspect any sort of workpiece.

[0054] It should be understood that the specific use examples given above
are by no means limiting, and that those of skill ip the art will
appreciate that the sensing apparatuses 10, 10', 10'', 10''', 110 may be
useful in an unlimited number of fields.

[0055] Referring to FIG. 10, another embodiment of a sensing apparatus 400
in accordance with the present invention is now described. The sensing
apparatus 400 is used to sense, or determine, a variety of properties of
a target 402, including, for example, the dimensions of the target, the
material composition of the target, and the thickness of the target.

[0056] The sensing apparatus 400 includes an excitation source 404,
illustratively a broadband optical source, configured to induce
ultrasonic waves in the target 402. The excitation source 404 is an
optical source and is illustratively coupled to the target 402 via an
optical fiber 406 having an end portion 408 in physical contact with the
target, although it should be appreciated that in some embodiments the
excitation source is not coupled to the target via an optical fiber but
rather radiates the target via free space. The excitation source 404
induces the ultrasonic waves in the target 402 by rapidly heating it. It
should be appreciated that in some applications, the excitation source
404 may be a coherent optical source (e.g. a pulsed laser), or a doped
fiber amplifier. In fact, in some applications, the excitation source 404
may be a pulsed laser having a spectral width that is inversely
proportional to the pulse duration.

[0057] A fiber optic interferometer 409 senses the induced waves and
generates target data based thereupon. In particular, the fiber optic
interferometer 409 comprises a plurality of optical couplers 416, 414,
422 and interconnecting optical fibers 430a-430e, 432a-432c arranged to
define a reference arm (430a-430e) and a measurement arm (432a-432c). A
probe segment 417 is coupled to portions of the reference arm 430c, 430d
and portions of the measurement arm 432a. The probe segment 417 has a
probe segment end 418 to be positioned adjacent the target 402b. An
optical path length adjustor 420 is coupled to portions of the reference
arm 430d, 430e. The optical path length adjustor 420 is illustratively a
piezoelectric body, although it should be understood that any suitable
optical path length adjustor or fiber stretcher may also be used.

[0058] A reference light source 410 is coupled to the reference arm
430a-430e and is configured to radiate light into the reference arm, and
onto the target 402 via the probe segment end 418. The reference light
source can be a laser source or doped fiber amplifier, as will be
appreciated by those skilled in the art. For example, the reference light
source may be a high gain erbium doped fiber amplifier with a 40 nm
bandwidth, centered around a wavelength of 1550 nm.

[0059] An optical power detector 412 is coupled to the reference arm
430a-430e and is configured to receive light from the reference light
source 410 reflected by the target 402 into the probe segment end 418.

[0060] The plurality of optical couplers 416, 414, 422 includes a first
optical coupler 416 coupling portions of the reference arm 430c, 430d to
portions of the measurement arm 432a and the probe segment 417. A second
optical coupler 414 couples the first optical coupler 416 to the
reference light source 410 and optical power detector 412. A third
optical coupler 422 couples portions of the reference arm 430e to
portions of the measurement arm 432a-432c, which thereby provides a
differential output to the photodetector 424.

[0061] A controller 426 is coupled to the optical path length adjustor 420
and is configured to adjust an optical path length of the reference arm
430a-430e to maintain a constant relationship with respect to an optical
path length of the measurement arm 432a-4320. The controller 426 may
adjust the optical path length of the reference arm 430a-430e based upon
the optical power detector 412 and/or the differential output provided to
the photodetector 424.

[0062] Thermal drifting may cause the length of the optical fibers within
the reference arm 430a-430e and the measurement arm 432a-432c to expand
and contract at different rates, which leads to the change of their
respective lengths. This is undesirable because it negatively affects the
accuracy of the sensing apparatus 400. The controller 426 helps rectify
this undesirable condition by adjusting the path length of the reference
arm 430a-430e using the optical path length adjustor 420. The matching of
the path length of the reference arm 430a-430e and the measurement arm
432a-432c by the controller 426 using the optical path length adjustor
420 to within 0.0025 in allows particularly accurate results.

[0063] Operation of the fiber optic interferometer 409 is now described. A
portion of the light radiated by the reference light source 410 is
radiated from the probe segment end 418 onto the target 402. This light
is then reflected from the target 402 back into the probe segment 417 via
the probe segment end 418, through the first optical coupler 416, through
the second optical coupler 414, and into the optical power detector 412.
The optical power detector 412 measures the optical power reflected from
the target 402, and due to the arrangement of the optical couplers 416,
414, 422, only the optical power reflected from the target. That is, the
optical couplers 416, 414, 422 are arranged such that the light directly
emitted by the reference light source 410 does not reach the optical
power detector 412, and only the light reflected from the target 402
reaches the optical power detector.

[0064] A portion of the light radiating from the reference light source
410 is conducted through the reference arm 430a-430e by the arrangement
of optical couplers 416, 414, 422 and to the photodetector. Consequently,
the light reflected from the target 402 and a portion of the light
radiated by the reference light source 410 and conducted through the
reference arm 430a-430e will combine, and the superposition thereof is
detected by the photodetector 424.

[0065] The light reflected by the target 402 will typically undergo a
phase change due to the ultrasonic waves and resulting vibrations in the
target, and therefore will have a different phase than the light radiated
by the reference light source 410 and conducted through the reference arm
430a-430e, causing constructive and destructive interference to occur
therebetween. This interference therefore reflects a detection of the
sensed induced waves and can be analyzed in order to determine various
properties of the target, as will be appreciated by those skilled in the
art.

[0066] The controller 426 generates target data based upon the sensed
induced waves. To do so, a pulse from the excitation source 404 triggers
the start of a measurement cycle, performed by the controller 426, in the
time domain. Signal peaks observed by the controller 426 correspond to
the transmit time of surface waves from the point of excitation (that is,
the point of the target 402 on which the pulse from the excitation source
404 radiates) to the probe segment end 418. Since the distance between
the excitation point and the probe segment end 418 is known, the acoustic
velocity of the ultrasonic waves in the target 402 can be calculated. By
comparing this acoustic velocity to a table of acoustic velocity for
different materials, the material composition of the target can be
determined. It should be understood that the excitation source probe 408
and probe segment end 418 can be scanned to different locations on the
target 402, so as to gather information about many points of the target.

[0067] The controller 426 may include a processor and a memory cooperating
therewith. The memory may be volatile or non-volatile, and the processor
may be an integrated circuit, in some applications.

[0068] In addition, the controller 426 performs typical interferometric
calculations as known to those of skill in the art on the superposition
of the light radiated from the reference light source 410 and directed
through the reference arm 430a-430e and the light reflected by the target
402 to potentially determine the dimensions and/or the thickness of the
target.

[0069] Since a difference in the length of the path traveled by the light
reflected by the probe segment end 418 and the light radiated from the
reference light source 410 and directed through the reference arm
430a-430e will result in an additional phase difference therebetween, it
is desirable for the length of the reference arm 430a-430e and the length
of the measurement arm 432a-432c toi remain the same, or at least
for a constant relationship between the length of the reference arm and
measurement arm to be maintained. If a constant relationship between the
length of the reference arm 430a-430e and the measurement arm 432a-432c
is to be maintained, it is desirable for the difference in length to be a
desired multiple of the wavelength of the light radiated by reference
light source 410. The multiple used need not be an integer in some
embodiments, and need not be greater than one in some embodiments.

[0070] It should be appreciated that the optical path length adjustor 420
need not operate by physically changing a length of an optical fiber in
all embodiments. For example, the optical path length adjustor 420 may be
an adjustable delay line or phase modulator which can maintain a constant
phase relationship between the light reflected from the target 402 and
the light radiated by the reference light source 410 and through the
reference arm 430a-430e. The maintenance of a constant phase relationship
between the light in the reference arm 430a-430e and the measurement arm
432a-432c also helps to provide accurate results.

[0071] In some applications, the reference arm 430a-430e may even include
a free space element. One such embodiment is now described with reference
to FIG. 11. Here, the sensing apparatus 500 remains the same as the
sensing apparatus 400 of FIG. 10, except that the reference arm 530a-530e
includes a free space element. Here, the free space element is contained
within an adjustable lens arrangement 521. The reference optical fiber
530d terminates at a coupler on the first side of the adjustable lens
arrangement 521, and radiates reference light via free space and through
a first lens 523. The reference light then passed through a second lens
525, which focuses the light back into the reference optical fiber 530e
via another coupler. The distance between the first lens 523 and second
lens 525 is adjustable based upon input received from the controller 526.
This thereby allows adjustment of the length of the path of the reference
arm 530a-530e.

[0072] A method of operating a sensing apparatus is now described with
reference to the flowchart 550 of FIG. 12. The sensing apparatus includes
a fiber optic interferometer comprising a plurality of optical couplers
and interconnecting optical fibers arranged to define a reference arm, a
measurement arm, a probe segment coupled to the reference arm and the
measurement arm and having a probe segment end, and an optical path
length adjustor coupled to the reference arm.

[0073] After the start of the method (Block 551), the waves are induced in
a target using an excitation source (Block 552). Then, a probe segment
end is positioned adjacent the target (Block 553).

[0074] An optical path length of the reference arm is then adjusted via
the optical path length adjustor to maintain a constant relationship with
respect to an optical path length of the measurement arm, using a
controller (Block 554). The induced waves are then sensed using a
photodetector coupled to the controller (Block 555). Target data is then
generated based upon the sensed induced waves, using the controller
(Block 556). Block 557 indicates the end of the method.

[0075] It should be understood that the sensing apparatuses 400, 500
disclosed above may include an array of excitation sources 404, 504, and
an array of fiber optic interferometers 409, 509.

[0076] The sensing apparatuses 400, 500 disclosed herein are useful in a
wide variety of applications. For example, they may be useful in medical
imaging systems, for sensing and imaging body parts. For example, the
optical fibers 406, 408, 506, 508 of the excitation source and reference
light source 410, 510 may be inserted into arteries, in order to image
those arteries or measure the thickness thereof, or may be inserted into
a trachea in order to image various components of the digestive system of
a patient. Shown in FIG. 13 is an embodiment where the sensing apparatus
600 (similar to the sensing apparatuses 400. 500 disclosed above) is a
biological sensing device, and the target is an artery having an arterial
wall 650. Here, the controller will generate anatomical data about the
arterial wall 650, such as a thickness or density of the arterial wall.
Those skilled in the art will appreciate that any biological sample or
body part may be sensed using this sensing apparatus 600.

[0077] In addition, the sensing apparatuses 400, 500 may be used for
materials inspection. For example, small welds, or welds in inaccessible
places, may be inspected using the sensing apparatuses 400, 500. Wire
bonds in electronic devices may be inspected using the sensing
apparatuses 400, 500. Hydraulic lines, such as those used in avionics
systems of aircraft, or brake, lines of a motor vehicle, may be inspected
using the sensing apparatuses 400, 500. Shown in FIG. 14 is an embodiment
where the sensing apparatus 700 (similar to the sensing apparatuses
disclosed above) is a material inspection device, and the target 702 is a
workpiece having a weld 750 to be inspected. Here, the controller will
generate material data about the material weld 750, such as a thickness,
density, or composition of the weld 750. Of course, this material
inspection device 700 need not be limited to weld inspection and may be
used to sense or inspect any sort of workpiece.

[0079] Many modifications and other embodiments of the invention will come
to the mind of one skilled in the art having the benefit of the teachings
presented in the foregoing descriptions and the associated drawings.
Therefore, it is understood that the invention is not to be limited to
the specific embodiments disclosed, and that modifications and
embodiments are intended to be included within the scope of the appended
claims.